Update options/yaml_variables setters; apply FF stop_writing_builds_metadata

What does this MR do and why?

Context

Since some job metadata will be moved to new destinations (e.g. intrinsic data moved to ci_builds and immutable processing data to ci_job_definitions) we need to be able to use feature flags to read/write data to specific destinations.

options and yaml_variables are processing data that will be moved to ci_job_definitions.config.

This MR

1. Updates the options and yaml_variables setters with the following feature flag:

• stop_writing_builds_metadata (Roll out: #552065)

2. Fixes some areas affected by the FF as discussed in #552057 (comment 2728233011):
3. Refactors tests for write_metadata_attribute and read_metadata_attribute and moves them to metadatable_spec.rb.

• These are private methods that technically shouldn't be tested directly. So I've tested them under #options= and #yaml_variables= instead.

4. Fixes other Rspec tests as needed. Many of the tests were already fixed in anticipation of this change in !202317 (merged).

References

• Resolves last part of #552057 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #552057 (closed)